In its simplest form, coffee roasting comprises heating a single bean to a prescribed temperature at which point chemical reactions occur that transform the bean into the desired state of pyrolysis. These reactions occur in the last part of the heating cycle. Thus, the residence time at the terminal temperature is crucial because a difference in a few seconds in heat-history can have a significant effect on the taste of the coffee.
The problem is that it is difficult to design a roaster that will roast several hundred pounds of beans at one time and to roast every bean evenly. Whether the process for heat transfer is from convection, conduction, radiation, or some combination thereof, the heat is absorbed in the first few layers of a bean bed. Therefore, it is desirable to establish some means for equalizing bean temperature throughout the heating cycle so that when the final roasting temperatures are approached, all of the beans will be close to the same temperature during the pyrolysis process.
The prior art is replete with attempts to obtain roasting uniformity. For example, various approaches for roasting coffee are set forth in the United States patent of Schytil, U.S. Pat. No. 2,857,683.
In the aforementioned prior art processes, the heating time to reach critical temperatures were considered to be relatively unimportant. For example, prior art processes typically roasted coffee beans for periods of six to twenty minutes. However, in recent years, it has been found that coffee beans expand more and result in lower roast bean density if the heating process is speeded up to where the total heating cycle is accomplished in as short a time period as possible consistent with acceptable product characteristics, preferably within 70-90 seconds. Further, it has been found that these light density beans, when ground, have increased extractable solids and wettability, thus yielding an increase in extractable solids. The result of fast roasting is that coffee processors can fill the traditional 16 ounce container with a much reduced weight of coffee that still results in an equivalent number of cups as 16 ounces resulting from a longer roasting process.
One approach to the more rapid roasting of coffee beans is disclosed in the U.S. patent of Brandlein et al., U.S. Pat. No. 4,737,376. As disclosed therein, the beans have a residence time within the roaster for a period of much less than three minutes and perhaps less than 1.5 minutes. During roasting, the beans are subjected to a flow of heated gas which passes upwardly through a first foraminated container at a mass flow rate of at least ten pounds of gas per pound of beans. In that process, the depth of the expanded bed is less than 50% of the diameter of the container. Further apparatus for the fluidized bed roasting of coffee is disclosed in the U.S. patent of Sivetz, U.S. Pat. No. 3,964,175. The Sivetz disclosure also contains a survey of prior art fluid bed roasters.
A more recent approach for obtaining a rapid uniform roast is disclosed in our copending U.S. patent application Ser. No. 07/463,557, which was filed on Jan. 11, 1990, entitled "Apparatus and Process for Conditioning Particulate Material" and which is incorporated herein in its entirety by reference.
In essence, our earlier invention comprises a chamber for receiving a charge of coffee beans. The chamber has a generally circular base and an upwardly extending divergent wall defining a segment of a cone with a central axis and closed bottom. The divergent chamber wall preferably forms an included angle with respect to a horizontal plane of between 40.degree. to 85.degree. and also defines a plurality of openings in a lower portion thereof. Means are provided for inducing a mass of heated fluid generally tangentially into the chamber to rotate the coffee beans about the central axis of the chamber and for maintaining the rotating material in a relatively densely packed or controlled state during the heating thereof. During the rotation of the coffee beans, the chamber is stationary, i.e., it does not rotate about its central axis, so that there is relative movement between the rotating material and the stationary chamber. In addition, there is also vertical and radial movement of the coffee beans with respect to the chamber. There is also horizontal shearing within the spinning bed caused by beans near the bottom of the chamber moving faster relative to the beans near the top of the chamber.
Coffee roasters in accordance with our earlier invention uniformly roast batches of coffee very rapidly with an efficient use of energy. They also provide conditioning, cooling, heating and roasting apparatus which are relatively flexible, competitively priced, relatively simple in operation, free of complexity and easy to operate and maintain. In addition, such roasters occupy a relatively small area and can be rapidly converted to operate under different conditions in a job shop type of operation while fulfilling most of the requirements for food processing. Such roasters are referred to hereinafter as controlled spinning bed roasters or spinning bed roasters and are distinguished from fluid bed roasters even though both use a heated fluid such as air for transferring heat to the beans.
The commercial roasting of coffee beans generates a relatively large amount of chaff and other debris during the roasting process. This chaff and other debris becomes entrained in the heating medium, i.e., hot gas, and is typically carried downstream from the roaster to a separate cyclone separator. There are several problems associated with this approach. For example, the mixture of hot gas and chaff is flammable and, as the chaff is further reduced in size as it is transferred to the separator, may form an explosive mixture. In addition, because the separator is located downstream from the roaster, ducting and fittings are required to connect the two. This ducting increases the internal surface area that is exposed to the hot gas and chaff and subsequently becomes fouled with condensed smoke and chaff particles. If this fouling is allowed to build up, a fire hazard is created. Hence, periodic removal is required and is typically accomplished by burning out the roaster, i.e., bringing the entire loop up to a high temperature (perhaps 750.degree. F.) and holding until all deposits are reduced to ash. The larger surfaces, such as the roaster to separator ducting and separate cyclone surfaces, require more energy to overcome losses to the environment, both during roasting and burnout, even though the ducts are insulated. Further, a torturous path through elbows tends to amplify local deposits. Associated with the increased ducting is also an increased internal volume of the roaster loop. As smoke is generated during the roasting process, the greater the volume of smoke retained in the loop, the greater the destructive energy should the smoke ignite and cause an explosion.
It has now been found that an improved apparatus in accordance with the present invention overcomes many of the problems associated with the prior art devices. For example, it is now believed that the improved apparatus disclosed and claimed herein minimizes the likelihood of fire and/or explosion by reducing surface area for fouling and decreasing internal volume, lowers energy requirements by minimizing surface area exposed to the ambient environment, facilitates the handling and removal of chaff from the hot gas, reduces space requirements because roaster and separator are now integrated, results in a simpler design, saves floor space and lowers machine cost because a separate cyclone with its attendant support structure, extra duct work, extra insulation and thermal expansion joints between it and the roaster are eliminated.